This invention relates to a photocurable resin composition, a method of producing a shaped object by photo-curing the composition, a vacuum casting mold as a shaped object, a vacuum casting method using the above mold, and a novel urethane acrylate suitable as a photocurable component of the composition. More specifically, it relates to a photocurable resin composition which has excellent dimensional accuracy with a small volume shrinkage factor when it is photo-cured and can provide moldings and stereolithographed objects such a vacuum casting mold, which have excellent flexibility, elastic recovery and mechanical properties; a method of producing a shaped object by photo-curing the composition; a vacuum casting mold which has excellent release properties and is capable of producing moldings having excellent dimensional accuracy at a high productivity; a vacuum casting method using the above mold; and a novel urethane acrylate.
Generally, a liquid photocurable resin composition is widely used as a coating material, photoresist, dental material or the like. In recent years, particular attention has been paid to a method of optically shaping a photocurable resin composition three-dimensionally based on data input into a three-dimensional CAD system.
As for optical stereolithography technology, JP-A 56-144478 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses an optical stereolithography for producing a stereolithographed object, which comprises the steps of supplying a required amount of controlled optical energy to a liquid photocurable resin to cure it into a thin layer, further supplying a liquid photocurable resin onto the layer, irradiating the resin with light under control to cure it into a thin layer to be laminated on the above thin layer, and repeating this procedure to produce a stereolithographed object. A basic practical method therefor is further proposed in JP-A 60-247515. Since then, various optical stereolithography technologies have been disclosed in JP-A 62-35966, JP-A 1-204915, JP-A 2-113925, JP-A 2-145616, JP-A 2-153722, JP-A 3-15520, JP-A 3-21432 and JP-A 3-41126.
A typical method of optically producing a stereolithographed object comprises the steps of selectively irradiating a surface of a liquid photocurable resin composition in a container with light from an ultraviolet laser controlled by a computer to cure it to a predetermined thickness so as to obtain a desired pattern, supplying one layer of the liquid photocurable resin composition onto the cured layer, irradiating it with light from the ultraviolet laser similarly to cure it to form a continuous and integral cured layer, and repeating this lamination procedure until a stereolithographed object having a final shape is obtained. This typical method is commonly and widely used. Since this method makes it possible produce an intended stereolithographed object having a very complex shape easily and in a relatively short period of time, it has been attracting much attention in recent years.
As the photocurable resin composition used as a coating material, photoresist, dental material or the like, curable resins such as an unsaturated polyester, epoxy (meth)acrylate, urethane (meth)acrylate or (meth)acrylic acid ester monomer to which a photopolymerization initiator is added, are widely used.
The photocurable resin composition used in the optical stereolithography includes those containing at least one photopolymerizable compound such as a photopolymerizable modified urethane (meth)acrylate-based compound, oligoester acrylate-based compound, epoxy acrylate-based compound, epoxy-based compound, polyimide-based compound, aminoalkyd-based compound, vinyl ether-based compound and the like as a main component(s) and a photopolymerization initiator. In recent years, JP-A 1-204915, JP-A 1-213304, JP-A 2-28261, JP-A 2-75617, JP-A 2-145616, JP-A 3-104626, JP-A 3-114732 and JP-A 3-1147324 have disclosed techniques for improving these substances.
It is required from viewpoints of handling properties, shaping speed, shaping accuracy and the like that the photocurable resin composition used in the optical stereolithography be a liquid having a low viscosity, have a small volume shrinkage when it is cured and provide a stereolithographed object having excellent mechanical properties when it is photo-cured. In recent years, with expanding demand and applications of optical stereolithographed objects, stereolithographed objects having high elongation, flexibility and elastic recovery properties in addition to the above characteristics are desired in some applications. For example, stereolithographed objects used for such applications as cushion materials, vacuum molding molds and the like are required to have high flexibility, high elongation and elastic recovery properties.
As a method for producing an optical stereolithographed object having flexibility, there is known one which comprises the steps of incorporating a thermally coherent polymer material comprising vinyl chloride resin powders and a plasticizer in a photocurable resin, curing the photocurable resin to obtain an optical stereolithographed object and thermally cohering the obtained stereolithographed object (see the above JP-A 3-104626). This method, however, has drawbacks that since a thermally coherent polymer is contained in the photocurable resin, the viscosity of the resin composition becomes high, so that handling properties and accuracy in shaping deteriorate. In addition, since a plasticizer is used, the resin composition is inferior in mechanical properties such as tear resistance and has such problems as transfer or oozing of the plasticizer to the surface of the resulting stereolithographed object. Therefore, satisfactory results are not achieved by this method.
Meanwhile, an urethane acrylate-based resin composition in which a caprolactone unit is bonded between urethane groups to improve tensile elongation is known (see JP-A 61-185522). A cured product therefrom has somewhat improved tensile elongation but is still unsatisfactory in flexibility.
A method of producing a molding by placing a mold in a vacuum atmosphere (reduced pressure atmosphere), injecting a curable resin such as polyurethane into the mold and curing the resin has been widely known as a vacuum casting method before the application of the present invention. In the case of the vacuum casting method, a molding having high dimensional accuracy and no air bubbles can be obtained smoothly because the resin component is injected into every corner of the mold swiftly and no air bubbles are contained into the resin as the resin component is injected into the mold under vacuum.
Further, as the vacuum casting method does not require large clamping force unlike an injection molding method, the strength of the mold is not required to be so high. Therefore, a silicon rubber-made vacuum casting mold which is excellent in such properties as flexibility, elasticity and release properties and allows a molding to be removed therefrom with ease has been widely used as a vacuum casting mold.
In this case, the production of a silicon rubber-made vacuum casting mold and a vacuum casting method using this mold are carried out by the following steps.
(1) A master model is fabricated mechanically and manually using an ABS resin or the like. PA0 (2) The master model fabricated in the above step is surrounded by an enveloping plate made from an acrylic resin or the like and a releasing agent is applied to the surface of the master model and the interior surface of the enveloping plate. PA0 (3) Curable silicon rubber which has been in advance vacuum defoamed as required is injected into the space formed by the master model and the enveloping plate and is cured by letting it stand, for example, at 50.degree. C. for 13 hours. PA0 (4) The cured silicon rubber is opened by cutting it with an operation knife as required and separated out from the master model to obtain a silicon rubber-made vacuum casting mold. PA0 (5) The silicon rubber-made vacuum casting mold obtained in the above step (4) is placed in a vacuum vessel and a liquid polymer material such as polyurethane which has been in advance vacuum defoamed as required is injected into the mold and cured or solidified. PA0 (6) The vacuum casting operation of the above step (5) is repeated as required to produce a plurality of vacuum cast moldings.
As is obvious from the above description, in the case of the above prior art method which uses a silicon rubber-made vacuum casting mold, a series of complicated production steps which take much time and labor such as the above-described steps (1) to (4) for the production of a master model and the production of a silicon rubber-made vacuum casting mold using the master model are required to produce a silicon rubber-made vacuum casting mold. In addition, since a master model is used, dimensional distortion tends to occur during the production of a master model and during the production (transfer) of a silicon rubber mold from the master model, and it is difficult to obtain a silicon rubber-made vacuum casting mold having excellent dimensional accuracy.
There is further proposed a method in which an ultraviolet light-curable resin is injected into a transparent silicon rubber mold formed from optically transparent silicon rubber which is curable at room temperature and cured under irradiation of ultraviolet light to produce a molding (JP-A 3-114711). However, this method also needs a series of complicated production steps which take much time and labor similar to the above-described steps (1) to (4) because a transparent silicon rubber mold must be fabricated using a master model. In addition, a dimensional distortion tends to occur during the production of a master model and during the production (transfer) of a transparent silicon rubber mold from the master model and it is difficult to obtain a transparent silicon rubber mold having excellent dimensional accuracy.